Add application/octet-stream MIME type to static_file_content response

In responses made to the static_file_content endpoint today, no value for the Content-Type header is defined. See this map definition. In turn, it does not appear that either Ring middleware or the underlying Jetty webserver add in a default value for the header either. For example, I see this on the wire for a request:

"GET /puppet/v3/static_file_content/modules/something/files/somefile.txt?environment=production&code_id=1234 HTTP/1.1\r\nAccept: binary\r\nAccept-Encoding: gzip;q=1.0,deflate;q=0.6,identity;q=0.3\r\nUser-Agent: Ruby\r\nHost: my-host:8140\r\n\r\n"

-> "HTTP/1.1 200 OK\r\n"

-> "Date: Fri, 02 Jun 2017 20:27:49 GMT\r\n"

-> "X-Puppet-Version: 5.0.0\r\n"

-> "Content-Length: 5\r\n"

-> "Server: Jetty(9.4.4.v20170414)\r\n"

-> "\r\n"

reading 5 bytes...

-> "test\n"

For good form, we should add a Content-Type header to the response. Since the content which is provided is translated as an input stream into raw bytes, the best Content-Type to use here is probably application/octet-stream.