Add test dependencies and expand coverage

[iamgollum]

Version 1.0 initial commit had no tests accompanied with it.

Add dependencies:

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.6.2</version>
    <scope>test</scope>
</dependency>

<dependency>
     <groupId>org.mockito</groupId>
     <artifactId>mockito-core</artifactId>
     <version>3.3.3</version>
<scope>test</scope>

Add tests at minimum for:

RabbitMQSourceConnector
HeaderSchema#toStructMap