getCustomerByCustomerNumber should be GET and return 200

[chrisjsimpson]

Currently (0b2a851)
the api call: /obp/v3.1.0/banks/BANK_ID/customers/customer-number (Get customer by customer number) expects a POST and returns a 201 created response.

The expected behaviour is:

It should be a GET request and the customer number be passed in the url.

https://apiexplorersandbox.openbankproject.com/#OBPv3_1_0-getCustomerByCustomerNumber