Allow '_' in header fields.

Technically anything defined as a 'token' by
http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html#sec2.2 should be
allowed, which includes !#$%^&*+-.`~| and probably others.  However
this is the only one that I have found in use.

http_parser.c

@@ -106,11 +106,10 @@ static inline int message_complete_callback (http_parser *parser)
 #define KEEP_ALIVE "keep-alive"
 #define CLOSE "close"
 
-
 static const unsigned char lowcase[] =
   "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"
   "\0\0\0\0\0\0\0\0\0\0\0\0\0-\0\0" "0123456789\0\0\0\0\0\0"
-  "\0abcdefghijklmnopqrstuvwxyz\0\0\0\0\0"
+  "\0abcdefghijklmnopqrstuvwxyz\0\0\0\0_"
   "\0abcdefghijklmnopqrstuvwxyz\0\0\0\0\0"
   "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"
   "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"

test.c

@@ -642,6 +642,31 @@ const struct message responses[] =
   ,.body= "hello world"
   }
 
+#define UNDERSTORE_HEADER_KEY 7
+  // shown by
+  // curl -o /dev/null -v "http://ad.doubleclick.net/pfadx/DARTSHELLCONFIGXML;dcmt=text/xml;"
+, {.name="underscore header key"
+  ,.type= HTTP_RESPONSE
+  ,.raw= "HTTP/1.1 200 OK\r\n"
+         "Server: DCLK-AdSvr\r\n"
+         "Content-Type: text/xml\r\n"
+         "Content-Length: 0\r\n"
+         "DCLK_imp: v7;x;114750856;0-0;0;17820020;0/0;21603567/21621457/1;;~okv=;dcmt=text/xml;;~cs=o\r\n\r\n"
+  ,.should_keep_alive= TRUE
+  ,.message_complete_on_eof= FALSE
+  ,.http_major= 1
+  ,.http_minor= 1
+  ,.status_code= 200
+  ,.num_headers= 4
+  ,.headers=
+    { {"Server", "DCLK-AdSvr" }
+    , {"Content-Type", "text/xml" }
+    , {"Content-Length", "0" }
+    , {"DCLK_imp", "v7;x;114750856;0-0;0;17820020;0/0;21603567/21621457/1;;~okv=;dcmt=text/xml;;~cs=o" }
+    }
+  ,.body= ""
+  }
+
 , {.name= NULL } /* sentinel */
 };