bug: Space character in Subject makes invalid query

[limoges]

First off, thanks for making this package available.

Now for the "bug".

Turns out that it is possible to have a space character in subject, for example "My little subject" is possible to create in at least Confluent Platform's Kafka schema registry.

When using *SchemaRegistryClient.GetSchemaVersion("My little subject") it will generate an http request like so:

GET /subjects/My+little+subject/versions

Unfortunately, this is not considered correct and schema registry will give us a not found. According to a blog post referred in this StackOverflow post, the package should encode space characters like so:

GET /subjects/My%20little%20subject/versions

This is because the URI standard, seems to expect:

• For URLs, a space in the path fragment has to be encoded with %20. A space in the query fragment can be encoded with +.

Since the subject is encoded into the URL path, a solution is to use url.PathEscape instead of url.QueryEscape for the cases where were escaping the path.

I'll open a pull-request associated with this issue,