erb.rb: Use script encoding instead of force_encoding

The original intention of introducing `_erbout.force_encoding`
in r21170 was:

- "returns a string in the same character encoding as the input string."
- "When the input string has a magic comment, however, it returns a string
  in the encoding specified by the magic comment."

And they are tested by test/erb/test_erb_m17n.rb well and this patch
passes the test.
Since magic comment is always added in ERB compiled code, using ''.dup
instead of String.new will set correct encoding without calling
force_encoding method.

The benchmark results are:

* Before

$ ./ruby benchmark/run.rb --matzruby=./ruby -m bm_app_erb
MatzRuby:
ruby 2.5.0dev (2017-05-26 skip-force-enc.. 58903) [x86_64-linux]
last_commit=Skip force_encoding in compiled code of erb
Ruby:

app_erb:
matz 0.715

* After

$ ./ruby benchmark/run.rb --matzruby=./ruby -m bm_app_erb
MatzRuby:
ruby 2.5.0dev (2017-05-26 skip-force-enc.. 58903) [x86_64-linux]
last_commit=Skip force_encoding in compiled code of erb
Ruby:

app_erb:
matz 0.672

And perf(1) results are:

* Before

$ sudo perf stat ./ruby benchmark/bm_app_erb.rb

 Performance counter stats for './ruby benchmark/bm_app_erb.rb':

        709.571746      task-clock (msec)         #    1.000 CPUs utilized
                 5      context-switches          #    0.007 K/sec
                 1      cpu-migrations            #    0.001 K/sec
             1,337      page-faults               #    0.002 M/sec
     3,088,936,521      cycles                    #    4.353 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
     4,849,564,282      instructions              #    1.57  insns per cycle
     1,027,042,087      branches                  # 1447.411 M/sec
        19,983,456      branch-misses             #    1.95% of all branches

       0.709747823 seconds time elapsed

* After

$ sudo perf stat ./ruby benchmark/bm_app_erb.rb

 Performance counter stats for './ruby benchmark/bm_app_erb.rb':

        693.494673      task-clock (msec)         #    1.000 CPUs utilized
                 7      context-switches          #    0.010 K/sec
                 1      cpu-migrations            #    0.001 K/sec
             1,316      page-faults               #    0.002 M/sec
     3,025,639,349      cycles                    #    4.363 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
     4,694,848,271      instructions              #    1.55  insns per cycle
       994,496,704      branches                  # 1434.037 M/sec
        19,693,239      branch-misses             #    1.98% of all branches

       0.693724345 seconds time elapsed

[fix GH-1147]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58904 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/erb.rb

@@ -280,7 +280,7 @@ class ERB
   # ERB#src:
   #
   #   compiler = ERB::Compiler.new('<>')
-  #   compiler.pre_cmd    = ["_erbout=String.new"]
+  #   compiler.pre_cmd    = ["_erbout=''.dup"]
   #   compiler.put_cmd    = "_erbout.<<"
   #   compiler.insert_cmd = "_erbout.<<"
   #   compiler.post_cmd   = ["_erbout"]
@@ -291,7 +291,7 @@ class ERB
   # <i>Generates</i>:
   #
   #   #coding:UTF-8
-  #   _erbout=String.new; _erbout.<< "Got "; _erbout.<<(( obj ).to_s); _erbout.<< "!\n"; _erbout
+  #   _erbout=''.dup; _erbout.<< "Got "; _erbout.<<(( obj ).to_s); _erbout.<< "!\n"; _erbout
   #
   # By default the output is sent to the print method.  For example:
   #
@@ -861,8 +861,8 @@ class ERB
   def set_eoutvar(compiler, eoutvar = '_erbout')
     compiler.put_cmd = "#{eoutvar}.<<"
     compiler.insert_cmd = "#{eoutvar}.<<"
-    compiler.pre_cmd = ["#{eoutvar} = String.new"]
-    compiler.post_cmd = ["#{eoutvar}.force_encoding(__ENCODING__)"]
+    compiler.pre_cmd = ["#{eoutvar} = ''.dup"]
+    compiler.post_cmd = [eoutvar]
   end
 
   # Generate results and print them. (see ERB#result)