Handle HTTP header parameters with empty values better

sdk/lib/io/http_headers.dart

Index: sdk/lib/io/http_headers.dart
diff --git a/sdk/lib/io/http_headers.dart b/sdk/lib/io/http_headers.dart
index 6b14ae2d6c89208388f3ed676f8f12b537e3de49..3769b3f0d04949bb1aa6e1ce5bbfee041ef9aa21 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -760,12 +760,12 @@ class _HeaderValue implements HeaderValue {
         }
         maybeExpect("=");
         skipWS();
-        if(done()) {
+        if (done()) {
           parameters[name] = null;
           return;
         }
         String value = parseParameterValue();
-        if (name == 'charset' && this is _ContentType) {
+        if (name == 'charset' && this is _ContentType && value != null) {
           // Charset parameter of ContentTypes are always lower-case.
           value = value.toLowerCase();
         }