Header keys are being lower-cased

### Expected Behavior

The headers are sent off as is, without text manipulation.

### Actual Behavior

The header is lower cased when sent off

However when using the Export Client Code feature, it is not lower-cased and requests work as expected.


### Reproduction Steps

1. Create a request
2. Add a header
3. Fire request and notice the change in capitalization between query builder and actual request
4. Generate Client Code and see how here it has not been modified

### Is there an existing issue for this?

- [X] I have searched the [issue tracker](https://www.github.com/Kong/insomnia/issues) for this problem.

### Additional Information

_No response_

### Insomnia Version

v2023.5.5

### What operating system are you using?

macOS

### Operating System Version

macOS Ventura - 13.5

### Installation method

Brew: https://formulae.brew.sh/cask/insomnia

### Last Known Working Insomnia version

_No response_

No source files or tests are named. Reproduce the issue in the request builder and compare the sent request's header casing with the generated client code; trace the header-handling entry point from there. Done means headers are sent with their original capitalization while generated client code remains correct.

Written by the indexing model from the issue text.