Ignore Content-Length in some cases:

fix #692

The value of Content-Length is not checked for limits
when the semantics of an HTTP response indicate that
the message has no body. For example, when status is 101.

include/boost/beast/http/impl/basic_parser.ipp

@@ -532,6 +532,11 @@ finish_header(error_code& ec, std::true_type)
     }
     else if(f_ & flagContentLength)
     {
+        if(len_ > body_limit_)
+        {
+            ec = error::body_limit;
+            return;
+        }
         if(len_ > 0)
         {
             f_ |= flagHasBody;
@@ -578,12 +583,20 @@ finish_header(error_code& ec, std::false_type)
         status_ == 204 ||        // No Content
         status_ == 304)          // Not Modified
     {
+        // VFALCO Content-Length may be present, but we
+        //        treat the message as not having a body.
+        //        https://github.com/boostorg/beast/issues/692
         state_ = state::complete;
         return;
     }
 
     if(f_ & flagContentLength)
     {
+        if(len_ > body_limit_)
+        {
+            ec = error::body_limit;
+            return;
+        }
         if(len_ > 0)
         {
             f_ |= flagHasBody;
@@ -876,12 +889,6 @@ do_field(field f,
             return;
         }
 
-        if(v > body_limit_)
-        {
-            ec = error::body_limit;
-            return;
-        }
-
         ec.assign(0, ec.category());
         len_ = v;
         f_ |= flagContentLength;