Return invalid params response as json

We didn't have a content-type before, so it was defaulting to
`application/octet-stream`, making browers unhappy.

src/clojars/web.clj

@@ -2,6 +2,7 @@
   (:require
    [cemerick.friend :as friend]
    [cemerick.friend.workflows :as workflows]
+   [cheshire.core :as json]
    [clojars.auth :as auth :refer [try-account]]
    [clojars.config :refer [config]]
    [clojars.errors :refer [wrap-exceptions]]
@@ -32,7 +33,8 @@
    [ring.middleware.content-type :refer [wrap-content-type]]
    [ring.middleware.defaults :as ring-defaults]
    [ring.middleware.flash :refer [wrap-flash]]
-   [ring.middleware.not-modified :refer [wrap-not-modified]]))
+   [ring.middleware.not-modified :refer [wrap-not-modified]]
+   [ring.util.response :refer [bad-request content-type]]))
 
 (defn try-parse-page
   "Will throw a targeted error if maybe-page doesn't parse as an integer."
@@ -127,8 +129,10 @@
       (let [explanation (pr-str (m/explain params-schema params))]
         (log/warn {:tag         :invalid-params
                    :explanation explanation})
-        {:status 400
-         :body   explanation}))))
+        (-> {:explanation explanation}
+            (json/encode)
+            (bad-request)
+            (content-type "application/json"))))))
 
 (defn clojars-app
   [{:as system

test/clojars/integration/web_test.clj

@@ -1,5 +1,6 @@
 (ns clojars.integration.web-test
   (:require
+   [cheshire.core :as json]
    [clj-http.client :as http]
    [clojars.db :as db]
    [clojars.errors :as errors]
@@ -10,6 +11,7 @@
    [kerodon.core :refer [fill-in follow-redirect follow press
                          session visit within]]
    [kerodon.test :refer [has text?]]
+   [matcher-combinators.matchers :as m]
    [matcher-combinators.test]
    [net.cgrand.enlive-html :as enlive]))
 
@@ -46,12 +48,18 @@
 (deftest invalid-params-are-rejected
   (is (match?
        {:status 400
-        :body "{:schema [:map-of :keyword :string], :value {:q {:a \"b\"}}, :errors ({:path [1], :in [:q], :schema :string, :value {:a \"b\"}})}"}
+        :body
+        (m/via json/decode
+               {"explanation"
+                "{:schema [:map-of :keyword :string], :value {:q {:a \"b\"}}, :errors ({:path [1], :in [:q], :schema :string, :value {:a \"b\"}})}"})}
        (search-request "q[a]=b")))
 
   (is (match?
        {:status 400
-        :body "{:schema [:map-of :keyword :string], :value {:q [\"b\"]}, :errors ({:path [1], :in [:q], :schema :string, :value [\"b\"]})}"}
+        :body
+        (m/via json/decode
+               {"explanation"
+                "{:schema [:map-of :keyword :string], :value {:q [\"b\"]}, :errors ({:path [1], :in [:q], :schema :string, :value [\"b\"]})}"})}
        (search-request "q[]=b"))))
 
 (deftest browse-page-renders-multiple-pages