fix(http): Properly URL-encode key and values during x-www-form-urlencoded POSTs

[theproducer]

This PR fixes keys and values that were not url-encoded in CapacitorHttp on iOS during application/x-www-form-urlencoded HTTP POST requests.

closes: #7840

[jcesarmobile]

Suggested change

	let allowed = CharacterSet(charactersIn: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._")
	let allowed = CharacterSet(charactersIn: "-._*").union(.alphanumerics)

This can be simplified with an union to .alphanumerics instead of typing them all
Also added * to match Android behavior, which uses URLEncoder.encode and according to the docs the * should remain the same:

The special characters ".", "-", "*", and "_" remain the same.
https://docs.oracle.com/javase/8/docs/api/java/net/URLEncoder.html#encode-java.lang.String-java.lang.String-

I've noticed that this is also encoding the keys, tested and web also encodes the keys, but Android doesn't encode the keys.
Not sure which one should be the correct behavior, but I think all the 3 platforms should behave the same way, so all of them should encode the keys or none should.

[jcesarmobile]

Another thing, on web and Android whitespace ( ) is converted to +, while on iOS it's getting converted into %20.

[theproducer]

Not sure which one should be the correct behavior, but I think all the 3 platforms should behave the same way, so all of them should encode the keys or none should.

According to MDN, keys being encoded is the correct behavior. However, changing it on Android could be seen as breaking, unless we categorize non-encoded keys as a bug (which I do).

What do you think?

[jcesarmobile]

yeah, it should be considered a bug and should be fixed