fix(http-executor): don't force application/json content type

[felamaslen]

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Description

• Fixes http-executor so as not to force the application/json content-type header
• This is necessary as you might want to specify a custom content-type header, such as application/vnd.api+json

Related #5824

Type of change

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• 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 not work as
  expected)
• This change requires a documentation update

Screenshots/Sandbox (if appropriate/relevant):

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How Has This Been Tested?

• Added a unit test on the http executor builder, asserting that a custom content type is passed through to the fetch function
• The test fails on previous commit, passes with the changes

Test Environment:

• OS:
• @graphql-tools/...:
• NodeJS:

Checklist:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

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[changeset-bot]

🦋 Changeset detected

Latest commit: ea363c0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@graphql-tools/executor-http	Patch

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[ardatan]

Thanks for the PR!
I left some comments then we can merge and release!

[felamaslen]

Thanks for the PR!
I left some comments then we can merge and release!

Thanks for review, have addressed 👍