Add test1180: Proxy request with -H "Proxy-Connection: Keep-Alive"

At the moment the test fails as curl sends two Proxy-Connection
headers.

tests/data/Makefile.inc

@@ -137,7 +137,7 @@ test1144 test1145 test1146 test1147 test1148 test1149 test1150 test1151 \
 test1152 test1153 test1154 test1155 test1156 test1157 test1158 test1159 \
 test1160 test1161 test1162 test1163 test1164 test1165 test1166 test1167 \
 test1168          test1170 test1171 test1172 test1173 test1174 test1175 \
-test1176 test1177 test1178 test1179 \
+test1176 test1177 test1178 test1179 test1180 \
 \
 test1188 \
 \

tests/data/test1180

@@ -0,0 +1,48 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+HTTP proxy
+</keywords>
+</info>
+
+# Server-side
+<reply>
+<data>
+HTTP/1.1 200 OK
+Server: Blafasel/5.0
+Date: Sat, 16 Jan 2021 14:48:30 GMT
+Content-Length: 12
+
+Bla bla bla
+</data>
+
+</reply>
+
+# Client-side
+<client>
+<server>
+http
+</server>
+<features>
+proxy
+</features>
+ <name>
+HTTP GET request with proxy and -H "Proxy-Connection: keep-alive"
+ </name>
+ <command>
+--proxy http://%HOSTIP:%HTTPPORT -H "Proxy-Connection: Keep-Alive" http://%HOSTIP:%HTTPPORT/1180
+</command>
+</client>
+
+<verify>
+<protocol>
+GET http://%HOSTIP:%HTTPPORT/1180 HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+User-Agent: curl/%VERSION
+Accept: */*
+Proxy-Connection: Keep-Alive
+
+</protocol>
+</verify>
+</testcase>