Set charset to utf-8 for httprequests.

New dart-sdk defaults to ISO-8859-1 (Latin 1) encoding when
encoding is not specified using response.headers.contentType. This
breaks our code, because we are always sending json encoding
objects containing UTF-8 encoded strings.
Merge pull request #21 from cleandart/fix-content-type

Set charset to utf-8 for httprequests.