INT-2874 Fix JSON/JMS Incompatibility

2.2.0 changed ObjectToJsonTransformr to add content-type to
simplify AMQP applications.

However, in a JMS environment, the DefaultJmsHeaderMapper attempted
to map (and failed) to map the header, emitting a warning log entry.

JMS does not allow '-' in property names.

Add code such that:

1. If the ObjectToJsonTransformer is configured to use a
content-type of an empty String (after trimming), suppress the
addition of the header to the output message. For consistency, if
the inbound message already has a content type header, and the transformer
is configured with an empty String, remove the header.

2. Change the DefaultJmsHeaderMapper to map the MessageHeaders.CONTENT_TYPE
header (content-type) to/from a JMS compliant property name (content_type).

INT-2874 PR Comments

Move constant to JmsHeaderMapper interface and rename.

Fix failing test (explicit set content-type to "").

INT-2874 Documentation Updates

Add docbook and schema documentation clarifying the behavior
of the transformer with respect to setting the `content-type`
header.

INT-2874 Polishing - Remove Header Removal

After further discussion, we decided to not remove an
existing header, if 'content-type' is set to "".

This is because there was no way to handle the case of
NOT adding a header when none present, while retaining a
header if it was already present.

added test for empty content-type attrib and no existing header (while merging)
Merge pull request #699 from garyrussell/INT-2874

* INT-2874: Fix JSON/JMS Incompatibility