fix: sentence-case doesn't allow upper-case characters in first word

This changes the `sentence-case` rule to also accept upper-case
characters in the first word, allowing the use of acronyms (e.g. "HTML",
"JSX") and words which include upper-case characters (e.g. "JavaScript")
at the beginning of a `sentence-case` input.

For example, with `'subject-case': [2, 'always', 'sentence-case']`, the
following commit messages were rejected (ESLint commit message style):

- `Fix: VAT note not properly displayed`
- `Upgrade: ESLint dev dependency`

Related: conventional-changelog#211

@commitlint/ensure/src/case.js

@@ -34,11 +34,8 @@ function toCase(input, target) {
 			return input.toUpperCase();
 		case 'sentence-case':
 		case 'sentencecase': {
-			const [word] = input.split(' ');
-			return `${toCase(word.charAt(0), 'upper-case')}${toCase(
-				word.slice(1),
-				'lower-case'
-			)}${input.slice(word.length)}`;
+			const [firstWord] = input.split(' ');
+			return firstWord.charAt(0).toUpperCase() + input.slice(1);
 		}
 		case 'lower-case':
 		case 'lowercase':

@commitlint/ensure/src/case.test.js

@@ -61,16 +61,16 @@ test('false for lowercase on sentencecase', t => {
 	t.is(ensure('sentence case', 'sentence-case'), false);
 });
 
-test('false for UPPERCASE on sentencecase', t => {
-	t.is(ensure('UPPERCASE', 'sentence-case'), false);
+test('true for UPPERCASE on sentencecase', t => {
+	t.is(ensure('UPPERCASE', 'sentence-case'), true);
 });
 
 test('true for Start Case on sentencecase', t => {
 	t.is(ensure('Start Case', 'sentence-case'), true);
 });
 
-test('false for PascalCase on sentencecase', t => {
-	t.is(ensure('PascalCase', 'sentence-case'), false);
+test('true for PascalCase on sentencecase', t => {
+	t.is(ensure('PascalCase', 'sentence-case'), true);
 });
 
 test('false for kebab-case on sentencecase', t => {