add some basic tests for clamp

these are expected to fail

* for Python 3, because the argument defaults (None) are incomparable
* for Python 2, because None<x for all ints and floats x, thus clamp(x)
  returns None

tests/test_mathutils.py

@@ -1,7 +1,9 @@
 
 from pytest import raises
-from boltons.mathutils import ceil, floor
+from boltons.mathutils import clamp, ceil, floor
+import math
 
+INF, NAN = float('inf'), float('nan')
 
 OPTIONS = [1618, 1378, 166, 1521, 2347, 2016, 879, 2123,
            269.3, 1230, 66, 425.2, 250, 2399, 2314, 439,
@@ -13,6 +15,24 @@ VALID_LOWER = 247
 VALID_UPPER = 2314
 VALID_BETWEEN = 248.5
 
+def test_clamp_examples():
+    """some examples for clamp()"""
+    assert 0 == clamp(0, 0, 1) == clamp(-1, 0, 1)
+    assert 0 == clamp(-1, lower=0)
+    assert 1 == clamp(1, 0, 1) == clamp(5, 0, 1)
+    assert 1 == clamp(5, upper=1)
+    assert 0.5 == clamp(7, upper=0.5)
+    assert 1 == clamp(7.7, upper=1)
+
+def test_clamp_transparent():
+    """clamp(x) should equal x beacause both limits are omitted"""
+    assert clamp(0) == 0
+    assert clamp(1) == 1
+    assert clamp(10**100) == 10**100
+    assert clamp(INF) == INF
+    assert clamp(-INF) == -INF
+    assert math.isnan(clamp(NAN))
+
 
 def test_ceil_basic():
     assert ceil(VALID_LOWER, OPTIONS) == VALID_LOWER