ruby typemaps matches to object even though SGMatrix is passed

[karlnapf]

Problems:

Passing matrices to methods that expect SGObject makes typemap match and produces a subsequent failure

require 'shogun'
svm = Shogun::LibSVM.new()
matrix = NMatrix.float(2,2)
svm.set_kernel(matrix)

resulting in TypeError: no implicit conversion of nil into String
See also swig/swig#1199

The error arises since SWIG will call a conversion method after the typemap matched.