fix: set default header

[nHuebner]

Context

When trying to do git operations with at least on premise BitBucket servers, the request could result in a 404.

Possible Solution

Set deafult header for isomorphic-git httpClient. Deafult behaviour for cli git command adds "Accept: /" header, to mimic this behaviour an defaultGitAcceptHeader will be set and merged with optional provided config headers.

Closes #2220

[filfreire]

@nHuebner heads up, I rebased the branch on top of latest develop.

Going to give it another try today and test to see if I can still reproduce the other problems. If not I think you should be able to merge it 👍