Puppet requests should include "real" MIME types

When a Puppet agent makes requests to a master, the values it uses in the Accept header do not correspond to industry-standard MIME types - although it really seems like they should.

For example, a request to the node endpoint includes:

GET /production/node/my-node?transaction_uuid=39794d9e-a0c8-4a5b-bcee-d20efa1555b5&fail_on_404=true

Accept: pson, b64_zlib_yaml, yaml, raw

For better HTTP spec compatibility, the request should look more like:

GET /production/node/my-node?transaction_uuid=39794d9e-a0c8-4a5b-bcee-d20efa1555b5&fail_on_404=true

Accept: text/pson, text/b64_zlib_yaml, text/yaml, application/x-raw

josh believes that this change would be fairly easy to make from the client side. Would need to call "model.get_format(format).mime" for each supported format to be added into the Accept header. Presumably, we'd have to have something on the server side to do the corresponding "decode" of the MIME type value back to the short-hand symbols that the server use to determine whether the client's request is valid. Not sure exactly what those changes would look like at this point.