Use same header format as curl for invoke command

[welteki]

Description

Change the header flag to accept key value pairs separated by a colon. e.g. "key: value".

The previous format is marked as deprecated but still supported. A warning is printed when it is used.

Fix an issues where the 'Content-Type' header was being overridden by the default value of the --content-type flag when the flag is not set explicitly.

e.g. when running faas-cli invoke -H "Content-Type=application/tar" the content type would be overridden with the default value text/plain.

Motivation and Context

• I have raised an issue to propose this change (required)

Align the format more closely with curl to make the invoke command as intuitive and familiar as possible.

How Has This Been Tested?

Changes are covered by unit test.

Verify headers can be set using the new format:

faas-cli invoke -H "foo: bar" <<< hello

Verify the deprecated format is still accepted and prints a warning:

faas-cli invoke -H "foo=bar" <<< hello
Warning: Using deprecated 'key=value' format for headers. Please use 'Key: Value' format instead.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[rgee0]

Could we pre-process header to replace = with : when a colon isn't present? Then we'd only need a header processing once.

We could also show the warning from the pre-processing step if a replace took place, and wrap a test around the pre-processor.

Alternatively, the header processing could be extracted as a function:

func processHeader(header, separator)(k, v){
...
}