Only lower-case ContentType's values if the name is 'charset'.

sdk/lib/io/http.dart

Index: sdk/lib/io/http.dart
diff --git a/sdk/lib/io/http.dart b/sdk/lib/io/http.dart
index d908338bc7a0ebc8776b3d93f4377b13b46f04eb..d00f6298d6937ec07a1026f292867637fb7d69fa 100644
--- a/sdk/lib/io/http.dart
+++ b/sdk/lib/io/http.dart
@@ -750,8 +750,9 @@ abstract class ContentType implements HeaderValue {
    * sub type. The charset and additional parameters can also be set
    * using [charset] and [parameters]. If charset is passed and
    * [parameters] contains charset as well the passed [charset] will
-   * override the value in parameters. Keys and values passed in
-   * parameters will be converted to lower case.
+   * override the value in parameters. Keys passed in parameters will be
+   * converted to lower case. The `charset` entry, whether passed as `charset`
+   * or in `parameters`, will have its value converted to lower-case.
    */
   factory ContentType(String primaryType,
                       String subType,