Handle empty strings in enums

[bensijons]

This PR fixes #236

In short, this XML:

<simpleType name="DriveTrain">
    <restriction base="string">
        <enumeration value="" />
        <enumeration value="Front Wheel Drive" />
        <enumeration value="Rear Wheel Drive" />
        <enumeration value="All Wheel Drive" />
        <enumeration value="Four Wheel Drive" />
    </restriction>
</simpleType>

Will generate the following broken go code:

This PR changes the implementation so it is generated like so: