add marshal for one element in enum

[carolinasolfernandez]

Generates a marshal function to assign the default value when in an enumeration type there is only one posibility

Example:

<complexType name="Person">
		<complexContent>
				<sequence>
					<element name="type" minOccurs="1" maxOccurs="1">
						<simpleType>
							<restriction base="string">
								<enumeration value="Human" />
							</restriction>
						</simpleType>
					</element>
				</sequence>
			</extension>
		</complexContent>
	</complexType>

type Person struct {
	Type         _anon1    `xml:"type"`
}

// May be one of Human
type _anon1 string

func (x *_anon1) MarshalText() ([]byte, error) {
	return []byte("Human"), nil
}

[droyo]

Hmm, what if the element is not provided at all, or is nillable? It doesn't seem correct to populate the value.