Handle #extract when carrier has symbolized keys

(Fixes #75)

lib/lightstep/tracer.rb

@@ -311,7 +311,7 @@ module LightStep
     def extract_from_rack(env)
       extract_from_text_map(env.reduce({}){|memo, tuple|
         raw_header, value = tuple
-        header = raw_header.gsub(/^HTTP_/, '').tr('_', '-').downcase
+        header = raw_header.to_s.gsub(/^HTTP_/, '').tr('_', '-').downcase
 
         memo[header] = value if header.start_with?(CARRIER_TRACER_STATE_PREFIX, CARRIER_BAGGAGE_PREFIX)
         memo

spec/lightstep_spec.rb

@@ -457,6 +457,33 @@ describe LightStep do
     span1.finish
   end
 
+  it 'should be able to extract from a carrier with string or symbol keys' do
+    trace_id = '123'
+    span_id = 'abc'
+
+    tracer = init_test_tracer
+
+    carrier_with_strings = {
+      'HTTP_OT_TRACER_TRACEID' => trace_id,
+      'HTTP_OT_TRACER_SPANID' => span_id,
+    }
+    string_ctx = tracer.extract(OpenTracing::FORMAT_RACK, carrier_with_strings)
+
+    expect(string_ctx).not_to be_nil
+    expect(string_ctx.trace_id).to eq(trace_id)
+    expect(string_ctx.id).to eq(span_id)
+
+    carrier_with_symbols = {
+      HTTP_OT_TRACER_TRACEID: trace_id,
+      HTTP_OT_TRACER_SPANID: span_id,
+    }
+    symbol_ctx = tracer.extract(OpenTracing::FORMAT_RACK, carrier_with_symbols)
+
+    expect(symbol_ctx).not_to be_nil
+    expect(symbol_ctx.trace_id).to eq(trace_id)
+    expect(symbol_ctx.id).to eq(span_id)
+  end
+
   it 'should handle concurrent spans' do
     result = nil
     tracer = init_callback_tracer(proc { |obj|; result = obj; })