fix: deep merge fetch options

[DennisBetawerk]

Problem
If a fetch instance is created with default header options, and that instance is then called with more headers the default headers are overwritten. The same goes for the param and query properties.

const createdFetch = $fetch.create({
  headers: {
    "Header-Default": "1",
  }
});

createdFetch("/some-url", {
  headers: {
    "Header-Request": "1",
  }
});

The outgoing request should have both the Header-Request and Header-Default headers, but will only contain the former.

Fix
Merging the headers, params and query properties of the FetchOptions object before fetching fixes this issue. A more generic deep merge approach was tried, but merging other properties such as the body makes little sense.

[codecov]

Codecov Report

Merging #243 (d174e6e) into main (253707a) will increase coverage by 3.56%.
The diff coverage is 100.00%.

❗ Current head d174e6e differs from pull request most recent head f346fa9. Consider uploading reports for the commit f346fa9 to get more accurate results

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   86.76%   90.32%   +3.56%     
==========================================
  Files           5        5              
  Lines         423      455      +32     
  Branches       67       73       +6     
==========================================
+ Hits          367      411      +44     
+ Misses         56       44      -12     

Impacted Files	Coverage Δ
src/fetch.ts	94.60% <100.00%> (+5.02%)	⬆️
src/utils.ts	95.19% <100.00%> (+2.04%)	⬆️