Increase BigMath.atan(x,prec) precision when |x|>0.5 (#320)

BigMath.atan(x, prec) is calculated with precision=prec+BigDecimal.double_fig when |x| <= 0.5.
But this safe margin was not properly applied when |x| > 0.5.

Author: tompng

lib/bigdecimal/math.rb

@@ -149,9 +149,9 @@ def atan(x, prec)
     x = -x if neg = x < 0
     return pi.div(neg ? -2 : 2, prec) if x.infinite?
     return pi / (neg ? -4 : 4) if x.round(prec) == 1
-    x = BigDecimal("1").div(x, prec) if inv = x > 1
-    x = (-1 + sqrt(1 + x**2, prec))/x if dbl = x > 0.5
-    n    = prec + BigDecimal.double_fig
+    n = prec + BigDecimal.double_fig
+    x = BigDecimal("1").div(x, n) if inv = x > 1
+    x = (-1 + sqrt(1 + x.mult(x, n), n)).div(x, n) if dbl = x > 0.5
     y = x
     d = y
     t = x