Add httpError to Expectation

[richardfuchs]

Is this repository still under development?
Are PRs welcome?

It would be nice to have "httpError" in this client.

Currently the following Expectation is implemented:

type Expectation struct {
	Request  RequestMatcher `json:"httpRequest,omitempty"`
	Response Response       `json:"httpResponse,omitempty"`
	Priority int32          `json:"priority,omitempty"`
}

but the MockServer also supports "httpError" link - the json for this would look somehow like this:

{
    "httpRequest": {
        "path": "..."
    },
    "httpError" : {
        "dropConnection" : true
    }
}

How can the "httpError" be added to the Expectation?

Sadly the solutions seems to be breaking:
The MockServer only accepts "httpResponse" or "httpError" in the Expectation
This means, that "httpResponse" or "httpError" must be omitted in the json

There is already a "omitempty"-flag on Response - however, this cannot be used properly.
The reason is, that the "omitempty" does not work on structs (like Response) - as structs do not have an empty value in go.
This problem could be solved using a pointer - as a pointer can be nil.

This solution could look like this

type Expectation struct {
	Request  RequestMatcher `json:"httpRequest,omitempty"`
	Response *Response      `json:"httpResponse,omitempty"`
	Error    *Error         `json:"httpError,omitempty"`
	Priority int32          `json:"priority,omitempty"`
}