fix!: drop requirement on URL/combine url and params

[JustinBeckwith]

This is a high risk change, please review carefully

I think this would fix #331, and fix #320. We currently use URL to parse the provided url, and then re-build the whole thing after merging querystring parameters. This was causing some issues for folks in #331 where the querystring parser doesn't really understand querystring properties with no values, and it was causing problems in #320 where CloudFlare Workers appear to not have a native URL available to them? That's neat!

This works around both problems by removing the use of URL, and doing a little hand crafting of the querystring. Right now I have this as a fix... I am open to conversations about the semverity of it though.

[codecov]

Codecov Report

Merging #338 into master will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
+ Coverage   95.53%   95.77%   +0.24%     
==========================================
  Files           6        6              
  Lines         672      663       -9     
  Branches      106      109       +3     
==========================================
- Hits          642      635       -7     
+ Misses         30       28       -2     

Impacted Files	Coverage Δ
src/gaxios.ts	98.18% <100.00%> (+0.64%)	⬆️

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[JustinBeckwith]

@bcoe this is the part of the change that wigs me out a bit. Previously, if you defined a querystring parameter both in the url and in params, we would only ever use one of them when forming the querystring. This strikes me as being ... incorrect. But it may be one of those bugs folks are relying on 🤷 My suspicion is that the risk of breaking runtime behavior is relatively low, because who relies on passing querystring parameters both in the url and in a params object.

[bcoe]

I'd like to do a quick review of axios, request, and fetch, see how they approach this -- I'll do so.

[bcoe]

I have tested axios and request, and convinced myself this is the right behavior.

[bcoe]

I believe this to be the right behavior, but I think we should consider taking it as a breaking change.

The danger I'm mainly worried about is the case where someone has done this:

?scopes=[some-scope]

For url, and also sets params = {scopes: [some other scope]}.