Strip quotes in detect-charset

[nathell]

What this PR does

RFC 7231 Section 3.1.1.1 allows the charset specified in the Content-Type header to be a quoted string. We want detect-charset to return unquoted charset names, so that they can be passed to, say, java.nio.charset.Charset/forName without postprocessing.

This PR fixes that, adds a test for a quoted charset, and rewrites t-detect-charset-by-content-type to use clojure.test/are to reduce repetition and read more nicely.

Implementation notes

• As noted above, I've taken the liberty of rewriting the salient test in terms of are, as I find it to boost readability. Some people disagree, however, and find are a bit too magical. I'm not sure what position of this project is (I haven't seen are anywhere else), but I can rewrite it if needed. Let me know!
• I wanted to use {starts,ends}-with? from clojure.string, but it would have broken compatibility with Clojure pre 1.8.0. So I resorted to Java interop.

Tests

I get one failure when running lein all test :all (stacktrace elided):

ERROR in (self-signed-ssl-get) (core_test.clj:298)
expected: (thrown? SunCertPathBuilderException (client/request {:scheme :https, :server-name "localhost", :server-port 18082, :request-method :get, :uri "/get"}))
2022-08-10 19:51:40,260 | INFO  | [main] | org.apache.http.impl.conn.InMemoryDnsResolver | Resolving foo.bar.com to [/127.0.0.1]
  actual: java.lang.IllegalAccessError: class clj_http.test.core_test$fn__5138$fn__5139 (in unnamed module @0x1b0db8b1) cannot access class sun.security.provider.certpath.SunCertPathBuilderException (in module java.base) because module java.base does not export sun.security.provider.certpath to unnamed module @0x1b0db8b1
ERROR in (self-signed-ssl-get) (core_test.clj:298)
expected: (thrown? SunCertPathBuilderException (client/request {:scheme :https, :server-name "localhost", :server-port 18082, :request-method :get, :uri "/get"}))
2022-08-10 19:51:40,260 | INFO  | [main] | org.apache.http.impl.conn.InMemoryDnsResolver | Resolving foo.bar.com to [/127.0.0.1]
  actual: java.lang.IllegalAccessError: class clj_http.test.core_test$fn__5138$fn__5139 (in unnamed module @0x1b0db8b1) cannot access class sun.security.provider.certpath.SunCertPathBuilderException (in module java.base) because module java.base does not export sun.security.provider.certpath to unnamed module @0x1b0db8b1

Doesn't look like related to my changes. I think it's because I'm on OpenJDK 11 and the tests presuppose JDK 8. Plain lein test comes out all green.