Change tanh to produce inexact values when approximating near 0.

Since the approximation is that (tanh x) = x for small x, previously
(tanh 1/10000000000000000000) produced an `exact?` answer, even though
this is an approximation.

Found by Typed Racket random testing:
http://drdr.racket-lang.org/54493/racket/share/pkgs/typed-racket-test/tr-random-testing.rkt

pkgs/racket-test-core/tests/racket/math.rktl

@@ -329,6 +329,7 @@
 (test 0 tanh 0)
 (test #t double=? tanh+1 (tanh 1))
 (test 1.0 tanh 20)
+(test #t flonum? (tanh (inexact->exact 1/100000000000)))
 
 #reader "maybe-single.rkt"
 (begin

racket/collects/racket/math.rkt

@@ -73,7 +73,7 @@
           [(real? z)
            (let loop ([z z])
              (cond [(z . < . 0)   (- (loop (- z)))]
-                   [(z . < . 1.29047841397589243466D-08) z]
+                   [(z . < . 1.29047841397589243466D-08) (exact->inexact z)]
                    [(z . < . 0.54930614433405484570D+00)
                     (define p0 -0.16134119023996228053D+04)
                     (define p1 -0.99225929672236083313D+02)