fix: reverts use of Headers for improved node compatibility

[krpeacock]

Description

Due to the use of Headers inside of Agent-JS, it introduced a nodejs compatibility issue. We can simply swap back with no impact on the functionality

Fixes #685

How Has This Been Tested?

Unit tests

Checklist:

• My changes follow the guidelines in CONTRIBUTING.md.
• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[github-actions]

size-limit report 📦

Path	Size
@dfinity/agent	88.83 KB (-0.02% 🔽)
@dfinity/candid	15.46 KB (0%)
@dfinity/principal	6.77 KB (0%)
@dfinity/auth-client	95.04 KB (0%)
@dfinity/assets	91.53 KB (0%)
@dfinity/identity	92.01 KB (0%)
@dfinity/identity-secp256k1	230.14 KB (-0.01% 🔽)

[ericswanson-dfinity]

Is there a note in the changelog to be reverted or amended?

[krpeacock]

@ericswanson-dfinity I don't think it's necessary - I'm keeping type support for someone to pass a Headers struct if they want to, but no longer using it by default in the implementation. Simply adding a note to the changelog should be adequate

[ericswanson-dfinity]

Simply adding a note to the changelog should be adequate

Did you mean to include such a note in this PR?