Fix WOperator error when the Jacobian is lazy

[YingboMa]

We want to use eltype(B) because it is always concrete.

[coveralls]

Coverage remained the same at 77.916% when pulling 083f5ae on myb/fix into 9876d03 on master.

[codecov]

Codecov Report

Merging #834 into master will decrease coverage by 5.86%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #834      +/-   ##
==========================================
- Coverage   80.22%   74.36%   -5.87%     
==========================================
  Files          88       88              
  Lines       29435    29361      -74     
==========================================
- Hits        23614    21833    -1781     
- Misses       5821     7528    +1707

Impacted Files	Coverage Δ
src/derivative_utils.jl	75.58% <100%> (-0.39%)	⬇️
src/tableaus/rkc_tableaus.jl	0% <0%> (-100%)	⬇️
src/caches/rkc_caches.jl	23.17% <0%> (-74.73%)	⬇️
src/perform_step/rkc_perform_step.jl	15.77% <0%> (-71.53%)	⬇️
src/caches/ssprk_caches.jl	43.93% <0%> (-38.16%)	⬇️
src/caches/low_storage_rk_caches.jl	57.41% <0%> (-32.13%)	⬇️
src/rkc_utils.jl	48.27% <0%> (-27.02%)	⬇️
src/alg_utils.jl	16.93% <0%> (-15.32%)	⬇️
src/perform_step/ssprk_perform_step.jl	69.81% <0%> (-15.22%)	⬇️
src/caches/low_order_rk_caches.jl	77.72% <0%> (-10.43%)	⬇️
... and 6 more

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