common: IsHex should be able to handle 0X prefixed strings

IsHex should also successfully decode strings prefixed with
0X instead of only 0x strings.

Also add tests generally for IsHex.

common/string_test.go

@@ -12,3 +12,21 @@ func TestStringInSlice(t *testing.T) {
 	assert.True(t, StringInSlice("", []string{""}))
 	assert.False(t, StringInSlice("", []string{}))
 }
+
+func TestIsHex(t *testing.T) {
+	notHex := []string{
+		"", "   ", "a", "x", "0", "0x", "0X", "0x ", "0X ", "0X a",
+		"0xf ", "0x f", "0xp", "0x-",
+		"0xf", "0XBED", "0xF", "0xbed", // Odd lengths
+	}
+	for _, v := range notHex {
+		assert.False(t, IsHex(v), "%q is not hex", v)
+	}
+	hex := []string{
+		"0x00", "0x0a", "0x0F", "0xFFFFFF", "0Xdeadbeef", "0x0BED",
+		"0X12", "0X0A",
+	}
+	for _, v := range hex {
+		assert.True(t, IsHex(v), "%q is hex", v)
+	}
+}