Refactor HTTP identifier names (API Change):

fix #171

Several names and  HTTP identifiers are renamed to be
more consistent, self-explanatory, and concise:

* "Fields" is a collection of HTTP header fields (rfc7230 section 3.2)
* "Header" is the Start Line plus Fields. Another way to look at it is,
  the HTTP message minus the body.
* `basic_fields` replaces `basic_headers`
* `fields` replaces `headers`
* `Fields` replaces `Headers` in template parameter lists
* `header` replaces `message_headers`
* `header::fields` replaces `message_headers::fields`

The changes are cosmetic and do not affect run-time behavior.

test/websocket/frame.cpp

@@ -68,7 +68,7 @@ public:
 
     void testFrameHeader()
     {
-        // good frame headers
+        // good frame fields
         {
             role_type role = role_type::client;
 
@@ -118,7 +118,7 @@ public:
             check(fh);
         }
 
-        // bad frame headers
+        // bad frame fields
         {
             role_type role = role_type::client;
 
@@ -211,7 +211,7 @@ public:
 
     void testBadFrameHeaders()
     {
-        // bad frame headers
+        // bad frame fields
         //
         // can't be created by the library
         // so we produce them manually.