SearchOptions {MaxResults: 0} doesn't work

## Describe the bug

`SearchOptions{ MaxResults:0 }` doesn't work. The API actually allows it and it's useful to get just the metadata.

## Expected behavior

No `Issue` returned if queried with `SearchOptions{ MaxResults:0 }`

```
// a piece of sample code
opts := jira.SearchOptions{ MaxResults: 0 }
issues, resp, _ := jiraCli.Issue.Search(`project=foo`, &opts)
fmt.Printf("NumIssues: %d, MaxResults: %d, Total: %d\n", len(issues), resp.MaxResults, resp.Total)

// expected output
NumIssues: 0, MaxResults: 0, Total: 12345

// actual output
NumIssues: 50, MaxResults: 50, Total: 12345
```

## Possible Solution

Remove `omitempty` from `MaxResults`

https://github.com/andygrunwald/go-jira/blob/f50cb07b297d79138b13e5ab49ea33965d32f5c1/issue.go#L500-L510

and tweak `if` conditions.

https://github.com/andygrunwald/go-jira/blob/f50cb07b297d79138b13e5ab49ea33965d32f5c1/issue.go#L1049-L1051

https://github.com/andygrunwald/go-jira/blob/f50cb07b297d79138b13e5ab49ea33965d32f5c1/issue.go#L1094-L1096

## Your Environment

* go-jira version (git tag or sha): v1.12.0
* Go version (`go version`): 1.14.2
* Jira type (cloud or on-premise): cloud
* Operating System and version: CentOS 8.1.1911