I use this proguard file:
-dontskipnonpubliclibraryclasses
-dontskipnonpubliclibraryclassmembers
!code/simplification/arithmetic,!code/simplification/cas
In Eclipse find org.apache.http.legacy.jar in ..sdk/platforms/android-23/optional.
Import it like external jar, check order/export and in proguard type:
-keep class org.apache.http.** { *; }
-keepclassmembers class org.apache.http.** {*;}
-dontwarn org.apache.**
-keep class android.net.http.** { *; }
-keepclassmembers class android.net.http.** {*;}
-dontwarn android.net.**