Parser concept, fixes:

A new concept Parser is introduced with routines to read from a stream
into the parser. This solves a problem with the old read interface where
messages must be default constructible and move assignable.

Parser fixes:

* Fix detect invalid reason-phrase octets
* Fix write_eof to set the 'complete' state on success
* Fix consider parse complete if eof received on empty body

WebSocket:

* Increase coverage

include/beast/core/impl/basic_streambuf.ipp

@@ -855,8 +855,11 @@ std::size_t
 read_size_helper(basic_streambuf<
     Allocator> const& streambuf, std::size_t max_size)
 {
-    return std::min<std::size_t>(max_size,
-        std::max<std::size_t>(512, streambuf.prepare_size()));
+    auto const avail = streambuf.prepare_size();
+    if(avail == 0)
+        return std::min(max_size,
+            std::max<std::size_t>(512, streambuf.alloc_size_));
+    return std::min(max_size, avail);
 }
 
 template<class Alloc, class T>