Conversion between FITSHeader and String

[aquatiko]

How can we convert a FITSHeader in a String format and vice-versa? It would be great to modify a Header by making a WCSTransform object, making the changes and then saving that to original FITS file. Using to_header from WCS.jl, we can get the header in string format but how can I write that to a FITSHeader?