change getHeaders() to be a Map, not a List

this makes it possible to use this library in conjunction with a
HttpURLConnection without playing stupid tricks with parsing the Strings
looking for colons.
test FormEncoding emits a Content-Type
rm extraneous newline
small changes from code review
don't use String.getBytes(Charset)

it's not available until JDK 6 and Android 2.3. also move Utils.UTF_8 to
the test package so we're not tempted to use it in the future.
Merge pull request #3 from mlc/map_headers

change getHeaders() to return a Map
Prepare for v1.1.0.
[maven-release-plugin] prepare release mimecraft-1.1.0
[maven-release-plugin] prepare for next development iteration