Math: don't allow implicit conversion of T to Complex<T>.

Brain failure #1.

src/Math/Complex.h

@@ -94,7 +94,7 @@ template<class T> class Complex {
          *      c = a + ib
          * @f]
          */
-        inline constexpr /*implicit*/ Complex(T real, T imaginary = T(0)): _real(real), _imaginary(imaginary) {}
+        inline constexpr /*implicit*/ Complex(T real, T imaginary): _real(real), _imaginary(imaginary) {}
 
         /**
          * @brief Construct complex number from vector

src/Math/Test/ComplexTest.cpp

@@ -97,8 +97,6 @@ void ComplexTest::construct() {
     Complex c(0.5f, -3.7f);
     CORRADE_COMPARE(c.real(), 0.5f);
     CORRADE_COMPARE(c.imaginary(), -3.7f);
-
-    CORRADE_COMPARE(Complex(2.0f), Complex(2.0f, 0.0f));
 }
 
 void ComplexTest::constructDefault() {