Fix GRADLE-2695: ClassFormatError introduced in 1.3

AsmBackedClassGenerator currently "overrides" the set method for any method
with the same name as the property, regardless of how many arguments it has.
The set method is intended to be an alias for the setter without starting with
"set" (e.g. instead of setProp(String), it would be prop(String)).

By accepting any method with the property's name, it is generating more set
methods than intended. If there is a prop(String, String) and a prop(String,
String, String) it will generate two prop(String) methods, resulting in a
ClassFormatError (as noted in GRADLE-2695).

This commit changes the overrideSetMethod() method to only perform the
override if the passed metaMethod is a one arg method.

See additional information in the [issue](http://issues.gradle.org/browse/GRADLE-2695).