Fix 2 mistakes in MatrixMarket format parsing

As per the full spec ( http://math.nist.gov/MatrixMarket/reports/MMformat.ps ):
- the header line starts with '%%MatrixMarket' (capitalized)
- the strings which specify array or coordinate formats are resp. 'array'
  and 'coordinate'
Merge pull request #3896 from huitseeker/issue/matrixmarket

BUG: io: fix 2 mistakes in MatrixMarket format parsing

As per the full spec (http://math.nist.gov/MatrixMarket/reports/MMformat.ps): The header line starts with '%%MatrixMarket' (capitalized). The strings which specify array or coordinate formats are resp. 'array' and 'coordinate'.