Floats and integers parsing fixes (#638)

* parser: fix scan of float with exp but no decimal
* decoder: validate leading zeros for decimals

unmarshaler_test.go

@@ -149,6 +149,11 @@ func TestUnmarshal_Floats(t *testing.T) {
 			input:    `-2E-2`,
 			expected: -2e-2,
 		},
+		{
+			desc:     "float exponent zero",
+			input:    `0e0`,
+			expected: 0.0,
+		},
 		{
 			desc:     "float fractional with exponent",
 			input:    `6.626e-34`,