Fix integer-valued array inputs

[timholy]

Currently this is broken (and it may be my fault):

julia> using Interpolations

julia> A = rand(1:20, 3, 3)
3x3 Array{Int64,2}:
 19  20   5
 17  19  14
 19   9   9

julia> itp = interpolate(A, BSpline(Quadratic(Flat)), OnCell);
ERROR: InexactError()
 in invert_column! at /home/tim/.julia/v0.4/Interpolations/src/b-splines/../filter1d.jl:38
 in _A_ldiv_B_md! at /home/tim/.julia/v0.4/Interpolations/src/b-splines/../filter1d.jl:21
 in A_ldiv_B_md! at /home/tim/.julia/v0.4/Interpolations/src/b-splines/../filter1d.jl:14
 in prefilter! at /home/tim/.julia/v0.4/Interpolations/src/b-splines/prefiltering.jl:38
 in prefilter at /home/tim/.julia/v0.4/Interpolations/src/b-splines/prefiltering.jl:26
 in interpolate at /home/tim/.julia/v0.4/Interpolations/src/b-splines/b-splines.jl:36
 in interpolate at /home/tim/.julia/v0.4/Interpolations/src/b-splines/b-splines.jl:39

Fix this once #34#36 and #36#37 get merged.

[timholy]

We also need to add tests for this issue.