Passing a dictionary to .n() now works on cos and sin. Issue 3538

Issue 3538.
Before,
```
>>> cos(x).n(subs={x:1+I})
cos(x)
```
Now,
```
>>> cos(x).n(subs={x:1+I})
0.833730025131149 - 0.988897705762865*I
```
The problem was that evalf_trig didn't work for complex arguments and was raising NotImplementedError.  But, `cos(1+I).n()` does evaluate.
The fix here was to make sure that substitution for x is occurring.