Bug 1055102 - Properly handle Unicode in Bagheera payloads; r=bsmedberg

It was observed that FHR was sending invalid JSON payloads to the
server. Specifically, JSON payloads contained invalid Unicode strings.

Investigation revealed that the culprint was CommonUtils.convertString()
silently swallowing high bytes. When the Bagheera client went to gzip
the JSON payload, the input buffer into gzip was missing high bytes.

This patch changes the bagheera client to UTF-8 encode strings before
gzip, thus ensuring all data is preserved. A corresponding change was
also added to the mock bagheera server implementation.

Alternatively, we could have changed CommonUtils.convertString() to
be high byte aware. However, many consumers rely on this function.
This patch is written with the intent of being uplifted and the change
performed is targeted at the specific problem.

Tests for Unicode preserving behavior have been added to both the
generic Bagheera client and to FHR. The latter test is arguably
not necessary, but peace of mind is a good thing, especially with
FHR.

See also bug 915850.

--HG--
extra : rebase_source : 4efddea7767c2e5f8cf19df247c3aba07c40eec6
extra : amend_source : ae3b6d89efa54fc9ed1794404476622946ad4b22