Add "accept" header to requests from proxy server

[kmcginnes]

The proxy server always assumes a response in JSON format, but that is not always the default response type.

This change makes it explicit that we want a JSON response by adding it to the headers:

headers: {
  "Content-Type": "application/x-www-form-urlencoded",
  "Accept": "application/json"
},

Validation

• Verified with Neptune, all languages
• Verified with https://beta.sparql.swisslipids.org

Related Issues

• Addresses [Bug] SPARQL access to queryid header leads failed search #393

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have run pnpm checks to ensure code compiles and meets standards.
• I have run pnpm test to check if all tests are passing.
• I have covered new added functionality with unit tests if necessary.
• I have added an entry in the Changelog.md.

[cubeddu]

LGTM