getHeaderLine() MUST return a string

[weierophinney]

Slight change: if the header does not exist, instead of returning null, return an empty string. This is more consistent with getHeader() (which returns an empty array in the same situation), and with the UriInterface (where absence of a value returns an empty string).

[weierophinney]

@Crell — please review!

[Crell]

+1!

[weierophinney]

@simensen and/or @pmjones — ready to merge!

[Tobion]

Are you sure there is no semantical difference between an absense of a header and the presence of a header with an empty value? I can image there are, e.g. a custom Debug header that is only checked if it is present like a boolean header similar to html attributes. Now there would be no way anymore to distinguish that.

[Tobion]

The comparison with UriInterface is not valid because in URI terms there is no difference between null and '', both mean it's not there. But for headers it's different.

[Tobion]

I figured there is also a hasHeader method which can be used for such "boolean" headers. So this change here is ok and does not prevent this use-case.