Add headers if not set

[detailyang]

Hello.
This PR is about the issue #60 for @SamMousa . It would add header if not present the specified header.
new option (-i) usage as the following:

 # add input header X-Foo *only* if it `X-Foo` doesnt exist
 more_set_input_headers -i 'X-Foo: howdy';

 # add output header X-Foo *only* if it `X-Foo` doesnt exist
 more_set_headers -i 'X-Foo: howdy';

@agentzh please review this PR 😁

[agentzh]

Will you remove these? Seems like you are not using the standard building process based on util/build.sh.

[agentzh]

@detailyang Please use the ngx-releng tool to check the coding style and other issues in your changes.

Also, please use the reindex tool to re-format your .t files.

[agentzh]

Line trailing spaces?

[agentzh]

@detailyang Thanks for the contribution! Please take care of the issues pointed out by my comments above. Thanks!

[detailyang]

@agentzh
This time i check the code style pointed out by you comments above carefully. Please review again 😁

Thanks!

[aleksandrs-ledovskis]

@agentzh / @detailyang

Is there any external help needed to bring this PR to completion?

[Fudoshiki]

what status?

[lmm-git]

What's the current status on this PR?

[ghost]

@agentzh What's the current status on this PR?

[jiricerhan]

Any progress there? I could really use this feature.

[janreges]

+1 ... it will be very helpful for us.