MutableMatrix.is_Piecewise

<em>Porblem:</em> Matrix.eye(3).is_Piecewise fails but S(Matrix.eye(3)).is_Piecewise does not because S() turns it into an
ImmutableMatrix. So the Mutable version does not have at least this (and maybe more) attributes set.

<em>Solution:</em> After a regular expression search through the code base, I found that is_Piecewise variable is declared in 
sympy/core/basic.py as 
<pre>is_Piecewise = False </pre>
which is never updated once it is initialized. 
As "S(What_ever_matrix).is_Piecewise" always returns False value, is it fine to add this line to sympy/matrices/matrices.py: 
<pre>is_Piecewise = False. </pre>
Which resolves the problem of the MutableMatrices at least for the above mentioned test case
After making the changes to the code, this the output:
<pre>>>> A=Matrix([(1, 2, 3), (2, 3, 4), (5, 6, 7)])
>>>A
Matrix([
[1, 2, 3],
[2, 3, 4],
[5, 6, 7]])
>>> A.is_Piecewise
False
>>> S(A).is_Piecewise
False
>>> </pre> 
All the tests were passed after making the changes.