Use double quotes only when needed for put_content_type_with_parameters.

library/network/protocol/http/src/http_header_modifier.e

@@ -213,6 +213,7 @@ feature -- Content related header
 			--| note: see `put_content_type_with_charset' for examples.
 		local
 			s: STRING_8
+			v: READABLE_STRING_8
 		do
 			if a_params /= Void and then not a_params.is_empty then
 				create s.make_from_string (a_content_type)
@@ -224,9 +225,14 @@ feature -- Content related header
 						s.append_character (' ')
 						s.append (nv.name)
 						s.append_character ('=')
+						v := nv.value
+						if v.has(' ') or v.has ('%T') or v.has ('=') then
 							s.append_character ('%"')
-						s.append (nv.value)
+							s.append (v)
 							s.append_character ('%"')
+						else
+							s.append (v)
+						end
 					end
 				end
 				put_header_key_value ({HTTP_HEADER_NAMES}.header_content_type, s)