fix(toMap): fix return type of keySelector #46

Job	Run time
Lint pull request (18.x)	1m 17s
Build and test pull request (14.x, cjs, es5)	2m 27s
Build and test pull request (14.x, cjs, es2015)	2m 28s
Build and test pull request (14.x, esm, es5)	2m 26s
Build and test pull request (14.x, esm, es2015)	2m 30s
Build and test pull request (14.x, umd, es5)	2m 5s
Build and test pull request (14.x, umd, es2015)	2m 9s
Build and test pull request (16.x, cjs, es5)	2m 21s
Build and test pull request (16.x, cjs, es2015)	2m 25s
Build and test pull request (16.x, cjs, esnext)	2m 21s
Build and test pull request (16.x, esm, es5)	2m 29s
Build and test pull request (16.x, esm, es2015)	2m 26s
Build and test pull request (16.x, esm, esnext)	2m 22s
Build and test pull request (16.x, umd, es5)	2m 8s
Build and test pull request (16.x, umd, es2015)	2m 8s
Build and test pull request (16.x, umd, esnext)	2m 6s
Build and test pull request (18.x, cjs, es5)	2m 24s
Build and test pull request (18.x, cjs, es2015)	2m 16s
Build and test pull request (18.x, cjs, esnext)	2m 15s
Build and test pull request (18.x, esm, es5)	2m 18s
Build and test pull request (18.x, esm, es2015)	2m 19s
Build and test pull request (18.x, esm, esnext)	2m 23s
Build and test pull request (18.x, umd, es5)	2m 6s
Build and test pull request (18.x, umd, es2015)	2m 1s
Build and test pull request (18.x, umd, esnext)	2m 0s
	56m 10s

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