BUG: Compare strings for equality, not identity.

In a script I found that calling MDS with dissimilarity='precomputed' was failing because strings are compared by identity. A quick way to reproduce the bug is this:

    from sklearn.manifold import MDS
    import numpy as np

    mds = MDS(dissimilarity=''.join(list('precomputed')))
    X = np.ones(10,10)
    pos = mds.fit_transform(X)

raises a

    ValueError: Proximity must be 'precomputed' or 'euclidean'. Got precomputed instead

The fix is really simple, but I can add a test case to it or an issue if you prefer that.