Add new Client::withLocation() method

[clue]

The withLocation(string $location): self method can be used to
return a new Client with the updated location (URI) for all functions.

Note that this is not to be confused with the WSDL file location.
A WSDL file can contain any number of function definitions.
It's very common that all of these functions use the same location definition.
However, technically each function can potentially use a different location.

$client = $client->withLocation('http://example.com/soap');

assert('http://example.com/soap' === $client->getLocation('echo'));

As an alternative to this method, you can also set the location option
in the Client constructor (such as when in non-WSDL mode).

Builds on top of #7 and #23, thank you @floriansimon1 and @pascal-hofmann!
Builds on top of #32