Add pre-conditions to reject invalid URL inputs

[mjul]

Add pre-conditions to check inputs when creating responses, ensuring that URLs look like URLs.

This improves the developer experience by giving a clear error message rather than an obscure stack trace from the underlying web server infrastructure.

Without this, if you accidentally wrote (created {:message "OK}) the framework would assume that the map is a URL and at a later point explode with a stack-trace like the one below.

With this change, it would give you a specific error in the call to created.

ERROR request UT005071: Undertow request failed HttpServerExchange{ POST /api/events/upload}
 java.lang.ClassCastException: class clojure.lang.PersistentArrayMap cannot be cast to class java.util.Collection (clojure.lang.PersistentArrayMap is in unnamed module of loader 'app'; java.util.Collection is in module java.base of loader 'bootstrap')
        at shadow.undertow.impl$set_headers$fn__19303.invoke(impl.clj:62)
        at clojure.lang.PersistentArrayMap.kvreduce(PersistentArrayMap.java:429)
        at clojure.core$fn__8525.invokeStatic(core.clj:6908)
        at clojure.core$fn__8525.invoke(core.clj:6888)
        at clojure.core.protocols$fn__8257$G__8252__8266.invoke(protocols.clj:175)
        at clojure.core$reduce_kv.invokeStatic(core.clj:6919)
        at clojure.core$reduce_kv.invoke(core.clj:6910)
        at shadow.undertow.impl$set_headers.invokeStatic(impl.clj:58)
        at shadow.undertow.impl$set_headers.invoke(impl.clj:56)
        at shadow.undertow.impl$ring__GT_exchange.invokeStatic(impl.clj:105)
        at shadow.undertow.impl$ring__GT_exchange.invoke(impl.clj:101)
        at shadow.undertow$eval19556$fn$reify__19563.handleRequest(undertow.clj:406)
        at io.undertow.server.handlers.BlockingHandler.handleRequest(BlockingHandler.java:56)
        at shadow.undertow.ShadowResourceHandler$1.handleRequest(ShadowResourceHandler.java:175)
        at shadow.undertow.ShadowResourceHandler.serveResource(ShadowResourceHandler.java:306)
        at shadow.undertow.ShadowResourceHandler.handleRequest(ShadowResourceHandler.java:139)
        at shadow.undertow.ShadowResourceHandler$1.handleRequest(ShadowResourceHandler.java:175)
        at io.undertow.server.Connectors.executeRootHandler(Connectors.java:387)
        at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:841)
        at org.jboss.threads.ContextClassLoaderSavingRunnable.run(ContextClassLoaderSavingRunnable.java:35)
        at org.jboss.threads.EnhancedQueueExecutor.safeRun(EnhancedQueueExecutor.java:2019)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.doRunTask(EnhancedQueueExecutor.java:1558)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1449)
        at java.base/java.lang.Thread.run(Thread.java:829)