Add UI for Http Request node headers

[Steve-Mcl]

fixes #3428

Types of changes

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

Proposed changes

• Add UI for headers
  
• Update built in help
• Add test - "should use ui headers"
  • verifies the RULES stated below
• Modify test - "should upload a file" to use UI entry for Content-Type instead of passing it in via msg.headers

RULES: msg.headers vs UI headers...

• If header "FOO": "BAR" is sent in msg.headers and is NOT present in the UI, then "FOO" is added to req.headers
• If header "FOO": "BAR" is sent in msg.headers and is "FOO": "BAZ" in the UI, then the "FOO" header is overwritten with "BAZ" in req.headers
• If header "FOO": "QUX" is sent in msg.headers and is "FOO": "" (i.e. no value) in the UI, then "FOO" is deleted from req.headers

NOTES...

• Header names are set in the case specified by the user
• UI Header names are Camel Case by default
  • If user wants to send content-type (lower-case) instead of Content-Type (Camel Case) then can either add it to msg.headers or add a UI entry set to "other" and name it content-type

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/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 (+1.5%) to 68.778% when pulling 5fda20c on Steve-Mcl:httpreq-headers-ui into 6a5c50f on node-red:dev.

[knolleary]

One minor help text tweak needed, but otherwise looks good.

Happy for you to merge once the help is updated.