Don't mangle user-provided searchParams string

[yocontra]

When providing a raw searchParams string, currently ky will pass it through new URLSearchParams() which has the result of mangling it in places where it is perfectly valid. A simple example is a querystring like a&b= (which is the output of qs.stringify({ a: null, b: '' }, { strictNullHandling: true })) is changed to a=&b= which semantically has a different meaning (a and b are both empty string).

This change is pretty simple, if a user provides a string don't mess with it - use it directly. This is a breaking change and should be released as a major if landed. I added one new test for a specific case with null values and also had to update other tests.

[sholladay]

The code looks good and I agree the behavior is arguably better. Though, by default all of the major parsers treat both ?foo and ?foo= as meaning that foo is an empty string and I'd strongly suggest not changing those defaults, otherwise it'll probably bite you at some point.