Bump to v0.6.1 #279

Job	Run time
Build `oauth-credentials` on MSRV (1.0.0)	12s
Build `oauth-credentials` on MSRV (1.6.0)	13s
Rustfmt	7s
Build `oauth-credentials` on MSRV (1.36.0)	26s
Test `oauth1-request-derive` (stable)	30s
Test `oauth-credentials` (stable, serde)	9s
Test `oauth1-request-derive` (beta)	42s
Test `oauth-credentials` (stable, std)	11s
Check with `-Z minimal-versions`	14s
Test (stable, js)	23s
Test `oauth1-request-derive` (nightly)	44s
Test `oauth-credentials` (beta, serde)	12s
Test (stable, js,derive)	44s
Test `oauth-credentials` (beta, std)	14s
Test (stable, js,hmac-sha1)	25s
Test `oauth-credentials` (nightly, serde)	18s
Test (beta, js)	34s
Test `oauth-credentials` (nightly, std)	15s
Test (beta, js,derive)	53s
Test (beta, js,hmac-sha1)	28s
Test (nightly, js)	27s
Test (nightly, js,derive)	39s
Test (nightly, js,hmac-sha1)	34s
Test (1.48.0, js)	1m 27s
Test (1.48.0, js,derive)	1m 27s
Test (1.48.0, js,hmac-sha1)	1m 23s
Test (stable, js, wasm32-unknown-unknown)	20s
	14m 11s

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