Built-in isSet function is returning false when a value is set

[bashbunni]

Describe the bug
Built-in isSet function is returning false when a value is set. This impacts GetBorderLeft and border rendering when setting a custom border.

To Reproduce
Add the source code to a test file, then run go test -run TestIsSet

Source Code

func TestIsSet(t *testing.T) {
	leftOnly := NewStyle().
		BorderStyle(Border{Left: "   new             "}).
		BorderForeground(Color("#FFA500"))
	got := leftOnly.isSet(borderLeftKey)
	if got != true {
		t.Fatalf("expected the left border to be set, got: %#v, but the left border is: %#v", got, leftOnly.borderStyle.Left)
	}
}

gives:

--- FAIL: TestIsSet (0.00s)
    style_test.go:550: expected the left border to be set, got: false, but the left border is: "   new             "
FAIL

Expected behavior
I would expect the borderLeftKey to be set

Additional context
This is likely related to other reported border and frame size issues.

[unamdev0]

Hi @bashbunni, I've created a PR for this,can you please check
#380