AP_Math: tests: make both arguments double

libraries/AP_Math/tests/test_math.cpp.3.o: In function `MathTest_IsEqual_Test::TestBody()':
test_math.cpp:(.text._ZN21MathTest_IsEqual_Test8TestBodyEv+0x1a0): undefined reference to `std::enable_if<std::is_floating_point<std::common_type<float, double>::type>::value, bool>::type is_equal<float, double>(float, double)'
collect2: error: ld returned 1 exit status

libraries/AP_Math/tests/test_math.cpp

@@ -84,7 +84,7 @@ TEST(MathTest, IsEqual)
     EXPECT_TRUE(is_equal(1.f, (float)(1.f - DBL_EPSILON)));
 
     // false because the common type is double
-    EXPECT_FALSE(is_equal(1., 1. + 2 * std::numeric_limits<double>::epsilon()));
+    EXPECT_FALSE(is_equal(double(1.), 1 + 2 * std::numeric_limits<double>::epsilon()));
 
     // true because the common type is float
     EXPECT_TRUE(is_equal(1.f, (float)(1. + std::numeric_limits<double>::epsilon())));