Folding ws between "Content-Type:" and "plain/text" leads to empty header (#96)

* Add testcase with folding whitespace between "Content-Type:" and "text/plain"

* Fix handling of newlines between Content-Type and the type itself

Author: sftse

resources/content_type.json

@@ -1217,5 +1217,12 @@
         ]
       ]
     }
+  },
+  {
+    "header": "\n text/plain\r\n",
+    "expected": {
+      "c_type": "text",
+      "c_subtype": "plain"
+    }
   }
 ]

src/parsers/fields/content_type.rs

@@ -329,6 +329,9 @@ impl<'x> MessageStream<'x> {
                     }
 
                     if next_is_space {
+                        if parser.state == ContentState::Type {
+                            continue;
+                        }
                         parser.state = ContentState::AttributeName;
                         self.next();