Fix Content-Type error

[kazuhitoyokoi]

I fixed #18 .

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

In the fix, node-red-nodegen automatically selects one appropriate Content-Type from Swagger definition. The priority of selection is as follows (The 1st, JSON format is the most appropriate for Node-RED flow).

• 1st: application/json
• 2nd: application/xml
• 3rd: text/csv
• 4th: text/plain
• 5th: multipart/form-data
• 6th: application/octet-stream

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage increased (+0.5%) to 74.247% when pulling 82aff50 on kazuhitoyokoi:master into 69ba8f9 on node-red:master.

[coveralls]

Coverage increased (+0.5%) to 74.247% when pulling 82aff50 on kazuhitoyokoi:master into 69ba8f9 on node-red:master.

[coveralls]

Coverage increased (+0.4%) to 76.821% when pulling a4007a2 on kazuhitoyokoi:master into 9b0d759 on node-red:master.