named arguments should be named

The main function 

function wavwrite(samples::Array, io::IO; Fs=8000, nbits=16, compression=WAVE_FORMAT_PCM)

is called in 

function wavwrite(samples::Array, filename::String; Fs=8000, nbits=16, compression=WAVE_FORMAT_PCM)

as if parameters Fs etc are positional, and not named
Merge pull request #5 from davidavdav/patch-1

named arguments should be named