URL: avoid double slash at the start of the path

[antkmsft]

Closes #5130.

Previous behavior:

  TEST(URL, LeadingSlashInPath)
  {
    Core::Url const u0("https://www.microsoft.com");
    Core::Url const u1("https://www.microsoft.com/");

    EXPECT_EQ(u0.GetAbsoluteUrl(), "https://www.microsoft.com");
    EXPECT_EQ(u1.GetAbsoluteUrl(), "https://www.microsoft.com");

    {
      auto url0 = u0;
      auto url1 = u1;
      url0.AppendPath("path");
      url1.AppendPath("path");
      EXPECT_EQ(url0.GetAbsoluteUrl(), "https://www.microsoft.com/path");
      EXPECT_EQ(url1.GetAbsoluteUrl(), "https://www.microsoft.com/path");
    }

    {
      auto url0 = u0;
      auto url1 = u1;
      url0.AppendPath("/path");
      url1.AppendPath("/path");
      EXPECT_EQ(url0.GetAbsoluteUrl(), "https://www.microsoft.com//path"); // <-- fixed in this PR
      EXPECT_EQ(url1.GetAbsoluteUrl(), "https://www.microsoft.com//path"); // <-- fixed in this PR
    }

    {
      auto url0 = u0;
      auto url1 = u1;
      url0.SetPath("path");
      url1.SetPath("path");
      EXPECT_EQ(url0.GetAbsoluteUrl(), "https://www.microsoft.com/path");
      EXPECT_EQ(url1.GetAbsoluteUrl(), "https://www.microsoft.com/path");
    }

    {
      auto url0 = u0;
      auto url1 = u1;
      url0.SetPath("/path");
      url1.SetPath("/path");
      EXPECT_EQ(url0.GetAbsoluteUrl(), "https://www.microsoft.com//path"); // <-- fixed in this PR
      EXPECT_EQ(url1.GetAbsoluteUrl(), "https://www.microsoft.com//path"); // <-- fixed in this PR
    }
  }

--
UWP CI is currently blocked until microsoft/vcpkg#35279 is fixed / microsoft/vcpkg#35116 is merged (#5181 would unblock)